Explanation:
A star with mass similar to that of our Sun will throw off its outer
gasses after fusion has stopped in its core. Possibly
the most visually spectacular of these planetary nebula is the pictured Ring
Nebula. The appearance as a ring is really an illusion of projection
- the nebula is
actually a spherical shell. At the center a blue dot is visible
which is the old core of the star,
known as a white dwarf. It is still not
known exactly how the star throws off the gas that becomes the nebula.

For more information on the Ring Nebula see The Electronic Universe Project's
write-up.